In recent years, a high pressure acid leaching method (HPAL method) using sulfuric acid has been attracting attention as a hydrometallurgical method for nickel oxide ore. Unlike pyrometallurgy, which is a conventional common refining method for nickel oxide ore, this method does not include a pyrometallurgical process, such as a reduction or drying process, but includes a consistent hydrometallurgical process, and thus is advantageous in terms of energy and cost. In addition, this method has another advantage that a sulfide containing nickel and cobalt (hereinafter, sometimes referred to as a nickel-cobalt mixed sulfide) whose nickel grade is improved up to approximately 50% by mass can be obtained.
This hydrometallurgical method for nickel oxide ore using the high pressure acid leaching method has, for example, the following steps. In other words, the hydrometallurgical method comprises: a leaching step wherein sulfuric acid is added to a slurry of nickel oxide ore and leached under high temperature and high pressure thereby to obtain a leached slurry; a solid-liquid separation step wherein multistage washing is applied to the leached slurry, whereby a leached residue is separated therefrom while a leachate containing an impurity element as well as nickel and cobalt is obtained; a neutralization step wherein the pH of the leachate obtained by the separation is adjusted to separate a neutralized precipitate containing impurity elements therefrom, whereby a post-neutralization solution containing zinc as well as nickel and cobalt is obtained; a dezincification step wherein hydrogen sulfide gas is added to the post-neutralization solution thereby to form a zinc sulfide, and the zinc sulfide is separated to obtain a mother liquor containing nickel and cobalt for nickel recovery; and a nickel recovery step wherein hydrogen sulfide gas is added to the mother liquor for nickel recovery to form a nickel-cobalt mixed sulfide, and the nickel-cobalt mixed sulfide is separated therefrom.
Here in the above-mentioned neutralization step of the hydrometallurgical method, for example, a leachate obtained from the solid-liquid separation step is fed into a neutralization tank and a calcium carbonate slurry is added thereto, thereby neutralizing the leachate, and an obtained hydroxide precipitate is solid-liquid separated, whereby a neutralized precipitate and a post-neutralization solution are obtained.
Furthermore, in the dezincification step, a post-neutralization solution is fed into a sulfurization reaction tank, and a sulfurizing agent, such as hydrogen sulfide gas or sodium hydrosulfide, is added thereto, thereby sulfurizing zinc, copper, and the like which are contained in the post-neutralization solution, and then solid-liquid separation is performed using a filter press or the like, whereby a mother liquor for nickel recovery, the mother liquor containing zinc sulfide, nickel, and cobalt, is obtained. (For example, refer to Patent Literatures 1 and 2.).
It should be noted that the nickel-cobalt mixed sulfide obtained by this hydrometallurgical method is further used as a raw material for purification to obtain electrolytic nickel and electrolytic cobalt, and therefore, in the above-mentioned dezincification step, the concentration of zinc (Zn) contained in a post-dezincification solution is required to be reduced to not more than 1 mg/L.
Furthermore, in the dezincification step, when a zinc sulfide formed is filtered and separated using a filter cloth, it is desirable to prevent the filter cloth from clogging up, thereby inhibiting a reduction in filtration velocity.
As a method for preventing a filter cloth from clogging up, there has been proposed a technique wherein a post-neutralization solution obtained by the above-mentioned neutralization step is adjusted to have a pH of 3.0 to 3.5, and also said post-neutralization solution is made to have a turbidity of 100 to 400 NTU, whereby a suspended solid comprising a neutralized precipitate and a leach residue is made to remain, and thus filterability is improved (For example, refer to Patent Literature 3.).
Furthermore, the operation in each of the steps is performed at a predetermined temperature and controlled under the optimal temperature. For example, in the neutralization step and the dezincification step mentioned above, the operation is performed at approximately 50 degrees C., and, each plant is equipped with equipment, such as a steam heater, capable of maintaining process water (including a valuable-metal-containing solution as an intermediate product, a post-neutralization solution, an overflow solution, and a post-dezincification solution) at an appropriate temperature.
However, at the time of a periodic inspection of a plant, removal of sludge staying at the bottom of each equipment (including tanks to store process water, such as a reaction tank, a thickening apparatus, and a storage tank; piping; and filters); cleaning of the equipment; replacement of breakage parts; and the like need to be performed, and therefore at least equipment to be subject to the inspection is drained of process water and made to be empty, and the temperatures of the equipment and the process water are reduced to an approximately atmospheric temperature (approximately 30 degrees C.).
Furthermore, in the start-up of a plant after completion of the periodic inspection, it takes approximately one day for a step of adding sulfuric acid to a slurry of nickel oxide ore and leaching under high temperature and high pressure (leaching step) to be in the 100% operating condition, and therefore the flow rate of process water during the above-mentioned start-up operation is unstable.
The dezincification step is particularly greatly affected by a temporary shut down due to such periodic inspection or the like, and thus it is difficult to simultaneously add hydrogen sulfide gas and a suspended solid as a seed crystal to process water whose flow rate and temperature are unstable. Furthermore, this implies that zinc as an impurity of high concentration is mixed into a nickel-cobalt mixed sulfide obtained after a sulfurization reaction in the nickel recovery step which follows the dezincification step.
Therefore, in the period of approximately one day required for the start-up, in order to reduce the concentration of zinc contained in a post-dezincification solution to not more than 1 mg/L, there is taken a measure wherein excessive hydrogen sulfide gas is added so that no zinc remains in a mother liquor for nickel recovery, the mother liquor being formed through the dezincification step. However, in this case, there is a problem that the zinc sulfide has a minute particle size, whereby a heavy load is imposed on a treatment of separating the zinc sulfide and a mother liquor for nickel recovery containing nickel and cobalt. Specifically, for example, a problem arises that a filter cloth (filter) used for the separation has a shorter lifespan.
Therefore, considering the above, there has been desired an operating method by which, plant start-up after completion of a periodic maintenance inspection or the like can be smoothly performed without imposing a load onto a filter cloth in the above-mentioned separation treatment of zinc sulfide, and the concentration of zinc in the post-dezincification solution can be effectively reduced, for example, even to a low concentration of not more than 1 mg/L.